Repeating apparatuses are installed in areas in which signals transmitted from a main transmitter are weak, amplify the signals transmitted from the main transmitter, and repeat the signals, thereby solving the problem of unstable reception and expanding the transmission range of the signals from the main transmitter.
FIG. 1 is a diagram illustrating the structure of a repeating system according to the related art in which different frequencies are used among repeating apparatuses.
When the main transmitter transmits signals of a frequency A, the repeating apparatuses (the first to fourth repeating apparatuses) repeat signals using different frequencies from the frequency A (frequencies B to E). In the repeating system according to the related art, since the repeating apparatuses use signals of different frequencies, a plurality of frequency bandwidths are used, and a large amount of frequency resources is consumed. As a result, the utilization of frequencies is lowered.
FIG. 2 is a diagram illustrating the structure of a repeating system according to the related art in which repeating apparatuses use the same frequency.
When the main transmitter transmits signals of a frequency A, the repeating apparatuses (the first to fourth repeating apparatuses) repeat signals using the frequency A. In order to repeat the signals through a common channel, the signals having the same frequency transmitted from the main transmitter and the repeating apparatuses should be individually identified.
When different signals in the same frequency bandwidth are used, it is difficult for the repeating apparatuses to remove an on-channel interference signal using their equalizers or other devices. When the signals transmitted from the main transmitter and the repeating apparatuses have a time delay that is more than a predetermined reference value, the equalizer cannot remove the delay signal. Therefore, in order to repeat the signals through the common channel, the same signal should be output from the repeating apparatuses and the main transmitter, and the time delay between the output signal from the main transmitter and the output signals from the repeating apparatuses should be small.
As the repeating apparatuses according to the related art, the following apparatuses are used: an RF amplification on-channel repeating apparatus, an IF conversion on-channel repeating apparatus, a SAW (surface acoustic wave) filter on-channel repeating apparatus, a modulation/demodulation on-channel repeating apparatus, and an equalization digital on-channel repeating apparatus. The problems of the above on-channel repeating apparatuses will be described with reference to FIGS. 3 to 7.
FIG. 3 is a diagram schematically illustrating the internal structure of the RF amplification on-channel repeating apparatus.
In the RF amplification on-channel repeating apparatus, a receiving antenna and an RF receiving module receive RF signals transmitted from a main transmitter, and an RF band pass filter filters signals in a predetermined band from the received RF signals. The filtered RF signals are amplified by a high power amplifying module, and then transmitted from a transmitting antenna through an on-channel.
FIG. 4 is a diagram schematically illustrating the internal structure of the IF conversion on-channel repeating apparatus.
In the IF conversion on-channel repeating apparatus, a receiving antenna and an RF receiving module receive RF signals transmitted from a main transmitter, and an IF down-converting module converts the received RF signals into intermediate frequency signals (hereinafter referred to as “IF signals”) on the basis of a reference frequency supplied from a local oscillator. The converted IF signals are filtered by an IF band pass filter. Then, an RF up-converting module converts the IF signals filtered by the IF band pass filter into RF signals on the basis of the reference frequency supplied from the local oscillator. The converted RF signals are amplified by a high power amplifying module, and then transmitted from a transmitting antenna through an on-channel.
FIG. 5 is a diagram schematically illustrating the internal structure of the SAW filter on-channel repeating apparatus.
In the SAW filter on-channel repeating apparatus, a receiving antenna and an RF receiving module receive RF signals transmitted from a main transmitter, and an IF down-converting module converts the received RF signals into IF signals on the basis of a reference frequency supplied from a local oscillator. The IF signals are filtered by a SAW filter, and an RF up-converting module converts the IF signals filtered by the SAW filter into RF signals on the basis of the reference frequency output from the local oscillator. The converted RF signals are amplified by a high power amplifying module, and then transmitted from a transmitting antenna through an on-channel.
The RF amplification on-channel repeating apparatus, the IF conversion on-channel repeating apparatus, and the SAW filter on-channel repeating apparatus respectively shown in FIGS. 3 to 5 cannot remove, as an example, noise and multipath signals generated due to a transmission line between the main transmitter and the on-channel repeating apparatus and a feedback signal generated due to low isolation of the transmitting and receiving antennas. As a result, characteristics of output signals are lower than those of input signals. In addition, the feedback signal generated due to low isolation of the transmitting and receiving antennas limits the transmission power of the repeating apparatus.
FIG. 6 is a diagram schematically illustrating the internal structure of the modulation/demodulation on-channel repeating apparatus.
In the modulation/demodulation on-channel repeating apparatus, a receiving antenna and an RF receiving module receive RF signals transmitted from a main transmitter, and an IF down-converting module converts the received RF signal into IF signals on the basis of a reference frequency supplied from a local oscillator.
A demodulation module demodulates the IF signals into baseband signals, and an equalization and FEC (forward error correction) decoding module removes, from the demodulated baseband signals, noise and multipath signals generated due to a transmission line between the main transmitter and the modulation/demodulation on-channel repeating apparatus and a feedback signal generated due to low isolation of the transmitting and receiving antennas.
An FEC encoding module performs forward error correction coding (FEC coding) on the output signal from the equalization and FEC decoding module, and a modulation module modulates the signal that is subjected to the FEC coding into an IF signal. An RF up-converting module converts the IF signal into an RF signal on the basis of the reference frequency supplied from the local oscillator. The converted RF signal is amplified by a high power amplifying module and then transmitted from the transmitting antenna through an on-channel.
In this way, the modulation/demodulation on-channel repeating apparatus solves the problems of the multipath signals and noise generated from the repeating apparatuses shown in FIGS. 3 to 5 using the equalization and FEC decoding module. However, the FEC encoding module and the FEC decoding module may increase a time delay from the range of a micro-second to a milli-second according to their operations. In addition, since the feedback signal, which has an ambiguity of a standard trellis encoder of the FEC encoding module, is not removed in the repeating apparatus, the transmission power of the repeating apparatus is limited.
FIG. 7 is a diagram schematically illustrating the internal structure of the equalization digital on-channel repeating apparatus.
In the equalization digital on-channel repeating apparatus, a receiving antenna and an RF receiving module receive RF signals transmitted from the main transmitter, and an IF down-converting module converts the received RF signals into IF signals on the basis of a reference frequency supplied from a local oscillator.
A demodulation module demodulates the IF signals into baseband signals, and an equalizing module removes, from the demodulated baseband signals, noise and multipath signals generated due to a transmission line between the main transmitter and the equalization digital on-channel repeating apparatus and a feedback signal generated due to low isolation of the transmitting and receiving antennas.
A modulation module modulates the baseband signals from which the noise, the multipath signals, and the feedback signal are removed into IF signals, and an RF up-converting module converts the IF signals into RF signals on the basis of the reference frequency supplied from the local oscillator. The converted RF signals are amplified by a high power amplifying module and then transmitted from the transmitting antenna through an on-channel.
In the equalization digital on-channel repeating apparatus, when a feedback signal having an electric field intensity that is higher than that of an input signal is received, the equalizing module cannot remove the feedback signal, and the equalizing module is diverged, which causes errors in the operation of the repeating apparatus.
The above-mentioned on-channel repeating apparatuses shown in FIGS. 3 to 7 have limitations in their capabilities to remove the feedback signals. As a result, the utilization of the repeating apparatuses is lowered, and a large investment is required to improve the utilization of the repeating apparatuses.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.